An optical frequency shifter does not perform photoelectric conversion on an input optical signal, but can shift a frequency (wavelength) of the optical signal itself. Thus, the optical frequency shifter is an important device in a wide range of fields such as an optical communication system and an optical measurement system.
The currently known optical frequency shifter uses single side band (SSB) modulation using an electro-optic phase modulator or a device using an acousto-optic effect.
In the method of using the acousto-optic effect, a diffraction phenomenon of light by a sound wave to be propagated is used. In this method, if light has a frequency which is in a high frequency region equal to or greater than 1 GHz, a wavelength of the sound wave is smaller than a wavelength of a light wave and diffraction does not occur. Thus, an upper limit of the frequency of the light is set to several hundred MHz.
In the SSB modulation using the electro-optic phase modulator, as illustrated in FIG. 14, an SSB modulator 1 is realized by combining a plurality of electric engineering phase modulators (low-frequency phase modulator 10, and high-frequency phase modulators 20a and 20b) and a Mach-Zehnder type optical waveguide circuit in many cases.
In Japanese Laid-open Patent Publication No. 2002-62516, one SSB modulation using an electro-optic phase modulator is provided and a modulation signal is only supplied from a single feeder circuit by using electric engineering crystals which form a polarization reversal structure, and thereby a frequency shift quantity of 1 GHz to 100 GHz is obtained.